Over the past years we focused our effort on development of metal-to-ligand charge transfer (MLCT) compounds as extrinsic luminescence probes for biochemistry and biophysics. These are based on d6 transition metal centers (Ru(II), Os(II), and Re(I)) chelated to ligands which are diimines, such a 2,2'-bipyridine (bpy), and other mono-or bidentate ligands. There are several approaches we have taken in order to develope this class of compounds as luminescence for biomolecules. We have presently isolated three amine-reactive and two sulfhydryl-reactive ligands that have been covalently linked to three different metal centers, and subsequently reacted with several classes of biomolecules. These probes are being used to develop new clinical chemistry assays, and are being distributed to other laboratories.